Interoffice trunk circuits for telephone systems



Aug. 7, 1923. 1,463,934 F. L. BAER INTEROFFICE TRUNK CIRCUITS FOR TELEPHONE SYSTEMS Filed June 18 1920 if; Ii

Irz' u EH'TU'P' Fred L. BEEF Elf g:

Patented Aug. 7, 1923.

nurse Ares FRED L. BAER, OF BERIIVYN, ILLINOIS, ASSIGNOR TO AUTOMIATIC ELECTRIC COMPANY,

OF CHICAGO, ILLINOIS, A

CORPORATION OF ILLINOIS.

Application filed June 18, 1920.

To all whom it may concern.

Be it known that 1, Final) L. BAnR a citizen of the United States of America, and a resident of Berwyn, Cook County, and State of Illinois, have invented certain new and useful Improvements in Interoflice Trunk Circuits for Telephone Systems, of

which the following is a specification.

My invention relates in general to inter- 10 ofiice trunk circuits for telephone systems, but more especially to trunk circuits for con necting automatic ofliccs with manual offices in the same system; and the object of the invention is the provision of an improved automatic to manual trunk circuit which is particularly useful for temporarily connectiug automatic and manual ofiices during a cut over, although if desired the trunk circuit may be used in permanently connecting such offices where automatic and manual systems serving the same territory are con solidated.

When a manual system is changed or cut over to an automatic system, the usual practice is to cut over one ofiice at a time and it necessarily follows that during the period of change, which may lastfor several years, both automatic and manual ofiices will be in operation. This renders it necessary to provide suitable trunk circuits for interconnecting the two different types of central offices, that is, automatic to manual trunks and manual to automatic trunks must be provided to handle the interoiiice traffic. This invention is concerned with the former class of trunks and may be considered as an improvement over former trunk circuits in that special means is provided for cancelling or absorbing certain digits in the telephone numbers of subscribers who are for the time being connected to a manual office which will ultimately be changed over to an automatic office.

This special means is a. switching device which I term a digit absorber, the operation and special function of which will be de scribed fully hereinafter, reference being had to the accompanying drawing, which is a circuit diagram of an interoflice trunk circuit embodying the principles of the invention.

Referring to the drawing, the reference characters 30 and 31 indicate the two con doctors of an interoflice trunk line. This Serial No. 389,818.

trunk line extends from the so called digit absorber E, which is located in an automatic office, to an answering jack J at an A operators position in a manual ofiice and 15 one of a group of similar trunk lines whlch are provided to handle the traflic from the automatic ofiice to the manual oflice. In order to give the automatic subscribers access to these trunk lines, they may be multipled in the banks of selector switches in the automatic office in the usual manner. Thus the trunk line shown in the drawlng may be reached by way of bank contacts 2, 3, and 4, which may be located in the bank of a first selector switch, being connected in multiple to similar contacts in the banks of the other first selectors. The automatic switches are of the well known Strowger vertical and rotary type, operated according to the well known two wire systern of control, and may be the type disclosed in U. S. Patent No. 1,281,112 granted @013. 8, 1918 to B. D. WVillis, and the digit absorber E, in addition to its digit absorbing function, also functions as a repeater to establish a holding circuit for any selector which is operated to establish connection with the trunk line associated therewith. 'This provision is in accordance with the usual practice and is made primarily to avoid the necessity of having three conductors between oflices.

As stated before, the answering jack J is located at an A. operators position in the manual office. The particular A operator in front of whom this jack appears may handle trunked calls exclusively, or the jacks associated with the incoming trunks from the automatic office may be distributed among a plurality of operators positions. In any case. the trunked calls are handled substantially like local calls originating in the manual ofiice and the operator, or operators, have uniform supervision. Cord circuits of the type illustrated in the fourth edition of American Telephone Practice, page 315, may be used. The group of relays indicated by the reference character F are the trunk relays, are located at the manual oiiice, and their functions and operations will be described fully in the course of the general explanation. The trunk equipment at the manual oliii'ze, it may be stated, forms no part of my invention, but is claimed in the pending application of WVillis, S. N. 389,820, filed June 18, 1920.

F or the purpose of explaining the oper' ation it will be assumed that a subscriber in the automatic oilice desires to obtain connection with a subscriber in the manual oflice, and the process of setting up the connection will be described. When the re ceiver is removed the calling line is extended by means of an individual line or trunk selecting switch to an idle first selector switch in the usual and well known manner, and the calling subscriber may then operate his calling device in accordance with the desired number.

The manual subscribers will have full and complete telephone numbers listed in the directory, in accordance with a comprehensive numbering system which has been previously worked out, and these telephone numbers are the numbers which will be used after the service is changed over completely to automatic. For the present the first digit is the only one which has to do with the control of an automatic switch, the remaining digits in any particular number being the ones which correspond to the location of the associated line in the multiple at the manual oifice.

hen the calling dial is operated for the first digit in the number the first selector is directively controlled to raise its shaft step by step until the wipers stand opposite the level which is reserved for the trunks extending to the desired manual office. At the end of the vertical movement of the selector the rotary trunk selecting movement is initiated automatically and the wipers are rotated step by step in search of an idle trunk line. Assuming that the trunk line shown in the drawing is the first idle trunk line, when the wipers arrive at bank contacts 2, 3, and i the rotary movement will cease and by the operation of the switching relay in the selector the calling line will be extended by way of the selector line wipers, bank contacts 2 and 4:, and conductors 16 and 18 to the upper and lower windings, respectively, of the double wound line relay 5 of the digit absorber E.

Upon the extension of the calling line as above described the line relay 5 is energized over the line circuit and closes a circuit for the slow acting release relay 6 at its armature 9. Upon energizing, relay 6 closes a holding circuit which may be traced from ground G by way of the left hand winding of induction coil R, normally closed contact springs controlled by armature 13, working contact of armature 10 and the said armature, release trunk conductor 17, test contact 3, test wiper of the operated first selector, and the winding of the switching relay in said selector to battery. This circuit serves to maintain the selector in operated position during the call and it may be added that a branch of the above circuit performs the same function for the subscribers individual line switch. It will be understood also that the ground potential on test contact 3 extends to multiple test contacts in the banks of other selector switches thereby rendering the trunk line busy.

The calling subscriber may now operate his calling device in accordance with the next digit of the desired number. As a result the line relay 5 is deenergized a plurality of times and sends a series of impulses to the slow acting relay 8, which attracts its armature and closes a circuit for the stepping magnet 23 of the digit absorber switch mechanism. The magnet is energized and attracts its armature causing the pawl carried thereby to engage the next notch in the ratchet wheel. .Relay 8, being slow acting, remains in operated position throughout the series of impulses, and only retracts its armature when the impulses cease. The circuit of the stepping magnet is then broken, its armature is retracted, and wipers 19 and 20 are advanced into engagement with the first contacts in their respective banks.

The calling subscriber may now operate his calling device in accordanc with the remaining digits in the desired number, and each time a digit is called th wipers 19 and 20 will be advanced one more step by the magnet 23. Assuming that the complcte telephone number consists of five digits, upon the calling of the final digit the wipers will be brought into engagement with the tfourth contacts in their respective banks, and wiper 19 will close a bridge across trunk conductors 30 and 31 as follows: from trunk conductor 30 by way of the winding of relay armature 15 and its working contact, right hand Winding of induction coil R. wiper 19 and bank contact engaged thereby, and conductor 21 to trunk conductor 31. It follows that relay *5 will be energized in series with relays and 34 at the manual office. the latter two relays being bridged across the trunk line in series with the battery. Upon energizing, relay 7 grounds the holding circuit direct at its armature 13 and thus cuts out the left hand winding of the induction coil B. At the manual exchange the relays 33 and 34 jointly clos a circuit for the answering or call signal lamp L. At the same. time relay 3 connects up a lead from the ringing machine ll M. Condenser 51 is of very low capacity, preferably about .005 M. F, and permits just enough ringing current to leak back through the receiver at the calling station to give the calling subscriber the usual. ringing current induction. An audible signal of this kind is customary in automatic systems and provided here for the sake Elli oi uni iornrity.

Hearing the signal, the calling subscriber will understand that the desired line is being signalled and will wait on the line, although, as matter of tact,the ringing: machine has nothing to do with the lling operation that has actually been -rior1ned.

.lt will be seen that although the interoFl'ice trunk line is connected with on calling the first digit ot the number, the answering lamp at the operators position is not lighted at once. but remains unlighted until th digit absorber has been stepped around by the calling of the remaining; digits tar enough to close the bridge across the trunk line. This eliminates the possibility of the flptl'lliOl attemptin to answer the call while the calling" subscriber is stillengaged in operating; his calling device. Conductor 21 may be shifted, it necessary, to some other contact in the bank of wiper 19, the contact to which it should be connected depending oi course on the numbering scheme in use.

lVhcn the operator sees the lighted answering lamp i she will understand that a call has been received over the associated trunk line and will respond by plugging into the jack J with an idle cord circuit. This lights the supervisory lamp in the cord circuit and energizes the sleeve relay 35. Relay upon energizing, prepares a locking circuit for relay 36 at its armature 44,

and at its armatures 43 and 45 disconnects relays 33 and 84 from the trunk line. This causes the answering lamp L to be extinguished. and also disconnects the ringing machin R M. At the same time relay 7 at the digit absorber E deenergizes andconnccts the left hand winding of repeating coil R into the holding, circuit again.

The operator will now throw her listening key and will note whether or not there is a discriminating" tone on the line. This operation is necessary of course only in case a charge is made for interoflice calls, which the operator is required to record. Any subscriber whose credit is poor and who is to be denied interofiice service on that account has his individual line switch circuit so arranged that a distinctive audible tone I is placed on the holding circuit whenever a call is attempted. This tone is transferred to the line at the digit absorber E by means ot the repeating coil R. and when heard by the operator is sufficient authority for her to refuse to complete the connection. Retcrence is again made at this time to U. S. Patent No. 1.2811152, granted Oct. 8, 1918, to D. Willis. which. shows and describes in full an arrangement of this kind. In order to proceed with the explanation we will assume-that no tone is heard by the operator when she comes in on the line, which means that the calling? subscriber is entitled to service. The operator will there fore obtain the desired number from the calling subscriber and will proceed to complete the connection with the other end of her cord circuit, either directly or with the assistance at a operator, depending on the particular type of manual exchange in use. The connection having; been established th operator will actuate key K momentarily and need pay no further attention to the call, nnless recalled. by the supervisory signals in the cord.

lVhen key K is operated a circuit is closed for relay 3? which, upon energizing, closes a circuit for relay at armature d9. Upon energizing; relay 36 locks itself at armature 47 and at relay 34. of relay 36, relay 37 connects its upper winding in of its armature 48. Condenser 52 is included in the upper conductor of the trunk line, normally prerentinp; current flow from the cord circuit; and it follows that when the condenser is shunted by the winding of relay 37, the said relay will be locked up by current flow over a circuit which ineludes the battery at the cord circuit, the supervisory relay at the cord circuit, and the relay 7 at the digit absorber E. Relay 7 is accordingly energized again to cut out the left hand winding of induction coil R, and at the cord circuit the supervisory relay is energized to put out the supervisory lamp. It may he mentioned also that relay 7 disconnects the condenser which is normally bridged around its winding, for the purpose of giving a good circuit forthe distinctive tone, when the tone is on the line.

The complete connection has now been established and when the called subscriber inthe manual oliice answers the conversation can begin.

When the subscribers are throuizh talking they will both replace their receivers. When the called subscriber hangs up the operator is given the usual di. connect supervision at the calling end of the cord circuit. W

o hen the calllnc subscriber hangs up the line relay 5 at the digit absorber E is permanently deenergized and falls back to break the circuit of the slow acting release relay 6. The latter relay then deenergizes also, the holding: circuit is broken. and the first selector and individual line switch are released. Relay 6, also closes a circuit for release, magnet 22 as follows: from the grounded armature 9 of the line relay by way of the resting contact ot' said armature, armature l1 and its resting contact, bank contact engaged by wiper 20 and the said wiper, conductor 2- and the winding of release magnet to battery. By the energrization ot tllQTQlQZlFQP'lEQHQl) the retaining pawl and driving pawl are both withdrawn from the ratchet wheel and the armature 4:6 prepares a circuit for in addition to closing the circuit shunt of condenser 52 by means wipers are restored to normal, the circuit of the release magnet being broken by wiper 20 when it is completely restored.

lVhen the digit absorber is released wiper 19 opens the bridge across trunk conductors 30 and 31, and as a result relay 7 and the supervisory relay in the answering end of the operators cord circuit are deenergized. The latter relay permits the answering supervisory lamp to be lighted and the operator now has a disconnect signal at both ends of her cord circuit. At the same time that the supervisory relay falls back the relay 37 deenergizes and thereby disconnects its upper winding from in shunt of condenser 52. Relay 36, however, remains locked up.

The operator may now pull down the connection and it she does so relays and 36 will be deenergized and all the apparatus willbe in normal position. However, in order to complete the explanation we will assume that another call is received over the same trunk line before the operator has had time to disconnect. Sometimes, especially during rush hours, the operators are rather slow at pulling down connections, and this may well occur. l Vhen the second calling subscriber connects with the trunk line the line and release relays 5 and 6 are energized as before, and when the calling of the number is finished the bridge including relay 7 isclosed across the trunk line by wiper 19 of the digit absorber. As a result a circuit is closed for relay 3st, which is now connected to the upper line conductor by way of armatures 43, 46, and 4.8, "for although relay 33 is disconnected, the lower line conductor goes to battery at the manual exchange, nevertheless, because the cord is still up. The supervisory relay at the cord circuit is included in the lower talking strand, which goes to the ungrounded pole of battery and it follows that relays 334i and 7 and the supervisory relay in the cord will be energized in series. This puts out the supervisory lamp and removes one of the disconnect signals. Relay 34 being now operated the circuit of? the answering lamp L is closed, but since relay 33 is at normal the lamp circuit will be closed through the interrupter I. This causes the lamp to flash which is more effective in recalling the operator than a permanent signal would be.

same condition as they were in the case previously described, when the trunk line was first taken for use, and the operator will proceed according to the usual routine.

Although it is not essential, I have shown a test jack J which is provided for the convenience of a switch board man or other employee in testing the trunk circuit. These test jacks may be mounted on the relay racks in the terminal room and afford ready access to the trunk circuits. When the jack J is connected with, a telephone or other testing device is bridged across the two upper springs of the jack while the two lower springs are short circuited. The latter operation energizes relay 32 which pulls up and opens the two sides of the trunk line, leaving the test telephone bridged across the line in the direction of the operator. The person testing can now talk to the operator and ascertain if the trunk line is working or not. Relay 32 also connects the un grounded pole of battery to the upper trunk conductor 80. Relay 6 at the digit absorber being at normal, relay 7 is now energized by current flow over the upper trunk conductor and at its armature 12 connects ground to the release trunk conductor 17. This renders the trunk line busy. It will be understood that if the trunk circuit should get out of order so that it would become necessary to establish a permanent busy condition, the lower springs of the test jack can be short circuited by means of any small metallic object/which may be left in place until the circuit is repaired.

Having described my invention, what I consider to be new and desire to have protected by Letters Patent will be pointed out in the appended claims.

What I claim is:

1. In a telephone system, means for connecting calling and called lines comprising a trunk line extending to an operators position, automatic switching mechanism whereby a calling subscriber can connect with said trunk line upon calling less than the total number of digits in a telephone number, and means individual to said trunkline operated responsive to the calling of.

the remaining digits in the number for signailing the operator.

2. In a telephone system, means for connecting calling and called lines comprising a trunk line extending to an operators position, automatic switching mechanism whereby a callingsubscriber can connect with said trunk line upon calling less than the total number of digits in a telephone number, and a digit absorber individual to said trunk line operated responsive to the calling of the remaining digits in thenumber for signalling the operator when the calling of the number is completed. I

3. In a telephone system, means for conreeaeee necting calling and called lines comprising a trunk line extending to an operators position, automatic, switching mechanism whereby a calling subscriber can connect with said trunk line upon calling less than the total number of digits in a telephone number, a digit absorber operated responsive to the calling of the remaining digits in the number for closing a bridge across said trunk lines, and a signal at the called operators position actuated when the bridge is closed.

4. In a telephone system, a trunk line connecting automatic and manual exchanges, a visual signal at the manual exchange, means including automatic switches whereby a callin# automatic subscriber can connect with said trunk line by calling the first digit in a telephone number, means in said trunk line responsive to each digit of the telephone number, said means closing a bridge at the outgoing end of said trunk line only when the final digit of the telephone number has been called for causing the operation of said signal.

In a telephone system, a trunk line connecting automatic and manual exchanges, means including automatic switches whereby a calling automatic subscriber can connect with said trunk line by calling the first digit in a telephone number, a digit absorber in said trunk line, a motor magnet for said digit absorber, means in said trunk line responsive to each digitfor completing a circuit for said motor magnet, a normally open brid e in said trunk line, a signal at the manual exchange operated when said bridge is closed, and means in said absorber for closing said bridge after the final digit has been called.

6. In a telephone system, a trunk line having one end terminating in terminals of an automatic switch at an automatic exchange and the other end terminating in a jack at a manual exchange, an automatic switch in said trunk line, means controlled by a calling subscriber for operating said first switch to seize said trunk line and for them operating said second switch through a plurality of positions, a bridge in said trunk line closed by said last switch only when it has been moved into a particular position, and a signal at the manual exchange operated when said bridge is closed.

Signed by me at Chicago, Cook County, Illinois, this li-t-h of June, 1920.

FRED L. BAER. 

